Your bra’s sole purpose in life is to support you and lift you up—kind of like a best friend for your breasts. But women don’t always choose their bras as carefully as their friends.

“A lot of women don’t take the time to find their right fit,” says Ashley Kelsch, founder and bra stylist of online luxury lingerie store, Teddies for Bettys. In fact, a May 2005 worldwide study by lingerie manufacturer Wacoal found that eight out of 10 women are wearing a bra that’s the wrong size. “If you’re going to wear the wrong size bra,” Kelsch says, “it will not be able to provide the necessary lift to your boobs. They will sag due to gravity's effect.”

Talk about a downer. Want tips on finding the perfect fit? Get them here. In the meantime, find out more fascinating facts about this wardrobe essential.

Certain bra sizes are interchangeable.

After trying on bras you discover you’re a size 34D in one brand and a 36C in another—what gives? Let’s set the record straight. “Bra cup size is a very subjective measurement to begin with,” says William Bruno, a board-certified plastic surgeon in Beverly Hills.“There is no standardization of cup sizes among bra manufacturers.” For example, different brands may define a "C" cup differently. “Because of this, women may wear a certain size from one place and feel this is the same size they should wear when shopping at all stores,” says Bruno. Rather than getting stuck on wearing a particular size, Bruno recommends focusing on finding the bra fit that works best for your bust. “It could be a ‘C’ at Victoria's Secret and a ‘B’ at their local department store,” he says.

Support doesn’t come from the straps.

Many women think bra straps provide most of the breast support. In truth, it’s the band that does the heavy lifting. “Our engineer found that almost 90 percent of the support comes from the band and not the shoulder straps,” says Lisa Zaccario, store manager at the Paramus, New Jersey location of Intimacy, which has boutiques throughout the U.S. that offer professional bra fittings and designer bras. “Most women will have better posture when wearing the proper-size bra because if the back band is loose, the weight of the breasts is shifted to the shoulder strap.” This can pull the body forward and cause you to slouch, especially if you have a full bust.

“The straps provide a little extra lift,” says Dawn Kenney, lingerie brand Wacoal’s national consultant manager. “But if the bra fits properly, you should be able to slide your straps off your shoulder and not see any loss of support.”

Underwire bras don’t cause cancer.

Chances are, you’ve gotten that scary viral email warning about underwire bras putting you at risk for cancer. “This is a myth that many believe,” says Jené Luciani, author of “The Bra Book: The Fashion Formula to Finding the Perfect Bra.” "The theory is that if the underwire bra is very tight and it’s riding up on your breast a bit and digging in, it would put pressure on your lymphatic system, causing it to not function properly. And that would give rise to a tumor. But medical science does not support this theory."

According to Breastcancer.org, a non-profit education organization: “There is no scientific evidence to support these rumors. Only one scientific study has looked at the link between wearing a bra and breast cancer. There was no real difference in risk between women who wore a bra and women who didn't wear a bra.”

It’s OK to sleep in a bra.

If you feel more comfortable tucking your girls into a bra at night, go for it. In fact, if you’re busty, you’re better off not leaving them to the mercy of gravity, even overnight. Allen Gabriel, M.D., associate professor of plastic and reconstructive surgery at Loma Linda University Medical Center in Loma Linda, Calif., says, "Wearing a bra to bed has benefits for women with large breasts or implants. It won't prevent sagging, but can possibly delay it in women over size B/C."

Just make sure that you’re going to bed in a bra that doesn’t have underwire and is not too tight. “Restrictive bras, especially those with underwire, can be harmful if the wire sits tight on skin and pinches it,” says Gabriel. “This can develop into a blister or a wound and even a long-term scar.”

Your bra size can and likely will change.

Your body is changing constantly, and so is your bra size. Here’s a shocker: “Bra size can change three to six times a year,” notes Zaccario. “So we ask our guests to stop by the store at least every six months to see if they’re wearing the proper size.” Your bra size can vary due to many factors, including weight gain or loss, medication, especially if it contains hormones, or menstruation. “In women going through menopause, breasts can enlarge and can grow up to two cup sizes,” explains Zaccario.

Your bra has an expiration date.

Even your bra needs a day off. Zaccario notes that the same bra should not be worn two days in a row. “The elastic should have at least one day off in between wearings,” she says. “This prevents the band from stretching out too quickly.” Zaccario also recommends washing bras with a lingerie-safe detergent in cold water.

Unlike diamonds, bras aren’t forever. If you have at least three bras in your wardrobe—and really, you should have at least that in rotation—expect your bra to last about a year to a year and a half. “The best advice is when you are wearing your bra on the tightest hook for more than three to four weeks, the bra has reached the end of its supportive life,” adds Zaccario. Bra funeral not required.

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